1. Field of the Invention
This invention relates to piezo electric transducers for generating an electrical signal related to the instantaneous velocity of the transducer.
2. Description of the Prior Art
Velocity responsive electromagnetic transducers of the seismic variety are widely used in vibration analysis. See U.S. Pat. No. 3,157,852. Velocity responsive eddy current devices also are employed to measure instantaneous velocity of vibrating bodies. See U.S. Pat. Nos. 3,521,158; 3,521,159. Such velocity responsive devices have limited sensitivity at vibration frequencies below ten cycles per second and above one thousand cycles per second.
Prior art accelerometer devices employing compressed piezo electric crystals have been employed to measure the instantaneous acceleration of a vibrating body. The prior art piezo electric crystal accelerometers develop an electrical charge which is proportional to the instantaneous acceleration. Such accelerometer installations are limited to low-sensitivity piezo electric crystals, e.g., sensitivities below 1,000 pico-coulombs per g. The instantaneous acceleration responsive electric charge is delivered by cable to a remote monitoring circuit for indicating the instantaneous acceleration by converting the "charge+ to a corresponding voltage. The limitation of sensitivity results from the fact that the more sensitive piezo electric crystals have a low natural frequency which tends to be excited in the crystal acceleration mode to yield a large output signal which saturates the charge amplifier. Thus to avoid amplifier saturation, the less sensitive piezo electric crystals have been employed, e.g., crystals having sensitivities of the order of 10 to 100 pico-coulombs per g.